Keyword "usually" - I have friends who pay month to month on a decent looking place well below value, they've been at the location for about 6-7 months now with no interest in signing a lease and neither does the landlord. The solution is still stupid, though. The first thing you do is pay your half of the rent and ask your landlord to give you 2-3 days extension for your roommate and if he doesn't show that you'll pay it for him. This does a few things for you, it shows your landlord that YOU are responsible [should you choose to remain living their] and that you're attempting to take responsibility for your roommate. If your roommate doesn't show up [who knows maybe something just happened] you pay that portion of the rent and than sell off some of his more expensive BUT REPLACEABLE items to make up what is owed. If you sell his TV its his own damn fault, but, if you sell his antique pocket watch from his great grandfather you're probably going to get your ass beat.. Once this is settled you use the time to look for a new roommate. Should your roommate reappear with an excuse that isn't incredibly amazing, you tell him you would like him to move out. If you signed a lease with both your names on it you're in a stickier situation but i'm sure you can find some way.

I can definitely see that working flawlessly